Such a LED light source is known from U.S. Pat. No. 7,081,722. The LED loads are LED arrays comprising series arrangements and possibly parallel arrangements of individual LEDs. The known LED light source comprises a rectifier for rectifying the low frequency AC supply voltage. A series arrangement comprising the N LED loads is connected to output terminals of the rectifier. During operation, a periodicDC voltage with a frequency 2f and an amplitude varying between zero Volt and a maximum amplitude is present between the output terminals of the rectifier. When the amplitude of the periodicDC voltage is zero Volt, none of the LED loads carries current. When the amplitude of the periodicDC voltage increases, a voltage is reached at which a first LED load starts carrying current. Similarly, when the amplitude of the periodicDC voltage increases further to a high enough value, a second LED load starts conducting.
A further increase of the amplitude of the periodicDC voltage subsequently causes the remaining LED loads to start carrying current.
When all the LED loads carry current, the amplitude of the periodicDC voltage increases further until the maximum amplitude is reached. After that the amplitude of the periodicDC voltage starts decreasing. While the amplitude decreases, the LED loads stop conducting current one by one in reversed order (first the Nth LED load stops conducting and the first LED load is the last to stop conducting). After the first LED load has stopped conducting, the amplitude of the periodicDC current decreases further to zero and then the cycle described hereinabove is repeated.
The known LED light source is very compact and comparatively simple. Furthermore, it can be directly supplied from a low frequency AC supply voltage source such as the European or North American mains supply. LED-utilization is defined a follows:
LED_Utilization (in case N=4)=(I_LED1_AVG/I_LED1_AVG*Vseg1+I_LED2_AVG/I_LED1_AVG*Vseg2+I_LED3_AV G/I_LED1_AVG*Vseg3+I_LED4_AVG/I_LED1_AVG*Vseg4)/Vstring_total wherein I_LED#_AVG is the average current through the LED load, evaluated over one period of the low frequency AC supply voltage, Vseg# is the LED load voltage, Vstring_total is the total voltage of all 4 LED loads.
The low LED utilization is caused by the fact that the different LED loads conduct current during time intervals of substantially different duration within a period of the periodicDC voltage. The Nth LED load carries current during a much shorter time interval than the first LED load. As a consequence, the first LED load carries a higher average current than the Nth LED load. The LED loads are generally formed by one or more LED packages comprising a number of multi-junction LED dies. In the manufacturing process, the packages that will be used in the first LED load are not discriminated from the packages that will be used in any of the other LED loads; therefore, all the packages have the same die size and package power capacity that has to meet worst case requirements. In this case, said requirements correspond to the use of the package in a first LED load (which, during operation, carries the highest average current of all the LED loads). However, most of the LED packages used in the LED light source are not used in the first LED load.